COMPARISON OF SIMPLE AND CHELATED AMBERLITE IR-120 FOR PRECONCENTRATION AND DETERMINATION OF Cu ( II ) FROM AQUEOUS SAMPLES

In the present study, the efficiency of simple and chelating Amberlite IR-120 with α-nitroso βnaphthol (IR-αNβN) and with 8-hydroxy quinoline (IR-8HQ) has been compared for the removal of Cu(II) from aqueous solutions. The chelation was confirmed using different characterization techniques like SEM, TGA and FTIR. A number of experiments were carried out in batch system to determine the effect of different parameters on adsorption of Cu(II) like pH, contact time and sample volume. The results showed occurrence of maximum adsorption at pH 7 in 10 min with adsorption capacity of 71.5 mg g at 298 K. The adsorption followed pseudo second order kinetic model among the four kinetic models applied. Maximum desorption from IR-8HQ was obtained with a mixture of 4.0 M HCl and 0.5 M HNO3. Furthermore, IR-8HQ was found to be most selective adsorbent among three adsorbents investigated. The developed preconcentration procedure was successfully applied to spiked tap water and real samples.


INTRODUCTION
With the rapid development of industries, the environmental pollution along with the water pollution containing many toxic metals is important research focuses in terms of natural equilibrium and health of all organisms.To improve the quality and quantity, it is important to monitor the amount of trace metal ions in the samples of environmental importance and it has become a more demanding issue.Industries that introduce trace metals in the environment are predominantly metal sanitizing, paints and tinctures, metal extraction, batteries discharge and industrial effluents [1][2][3].Copper is one of the heavy metals that tends to accumulate in the body and causes mucosal irritation, lung cancer, damage to capillary, kidney and liver and gastrointestinal symptoms such as diarrhea, abdominal pain, nausea, and vomiting [4].Sources of copper in drinking water are mining, ores processes and from industries like chemical, fertilizer, textile dyeing, printing, and paper and pulp mills [5].This makes the elimination of copper very important not only due its adverse effects on human health but also the probability of its reuse in many industries [6].
Different preconcentration procedures have been broadly used for removal of heavy metals and these include liquid-liquid or ion exchange extraction, coprecipitation, membrane separation and bio or chemical adsorption.Chelating ion exchange adsorption is one of the most nominal preconcentration methods due to its good mechanical stability, selectivity from a large aqueous volume, high enrichment factor, higher degree of interaction between adsorbate and adsorbent, high adsorption capacity for metal ions and regeneration of chelated resins.Therefore the demand of these chelated polymer resins for trace metal determination is increasing day by day.This is because; chelated polymer resins can be used for trace metal investigation mainly for water, biological and geological samples [7].
The chelated polymer resins with higher adsorption capacities can be prepared by taking chelating ligand of small size with populated functional group bound to a suitably cross-linked polymer.Due to the presence of different donor atoms on chelating ligands like oxygen, sulphur, nitrogen, these chelating ligands become more selective for adsorption of specific metal ion.The most widely used methods applied for preparation of chelated resins are either by simple adsorption of ligand on the polymer resin or by the intermediate functional group such as −N=N− by diazotization or −CH 2 − by methylene chloride reaction.Chelating polymers synthesized by covalent bonds are much more resistant to external effects than those by simple adsorption [15].
Amberlite IR-120, a vinyl benzene polymer has good chemical, physical and thermal stability.It has high exchange capacity and good ion exchange kinetics.These specific characteristics make it a good resin for use as a solid support.In the present study Amberlite IR-120 has been functionalized with α-nitroso β-naphthol and 8-hydroxy quinoline for preconcentration and determination of Cu(II) from aqueous samples and the results have been compared with the simple Amberlite IR-120.

EXPERIMENTAL Instruments
For characterization of the adsorbents used for preconcentration, scanning electron microscope model JEOL-JSM-5910 (Japan), Perkin Elmer Diamond TG/DTA and FTIR spectrophotometer Pretige 21 Shimadzu (Japan) were used.For the analysis of Cu(II) Perkin Elmer AAnalyst 200 (USA) atomic absorption spectrophotometer was used.

Reagents and chemicals
All chemicals and reagents used in this work were of analytical reagent grade purity and were manufactured by Merck, Germany.Chelating agent 8-hydroxy quinoline was obtained from Scharlau Chemie, USA, while Amberlite IR-120 was obtained from Across Organics, Belgium.

Solution preparation
Stock 1000 µgmL -1 copper standard solution was purchased from Merck.Working standards were prepared from this solution by dilution.Britton Robinson buffer solutions of pH 2-8 were prepared according to standard procedure [16] and were used for the adjustment of solution pH.

Chelation of Amberlite IR-120
Chelation of Amberlite IR-120 was done according to the procedure given in literature [15] with little modifications.20 g of Amberlite IR-120 (acid washed) was added to 80 mL mixture (3:5) of conc.nitric acid and sulphuric acid and was stirred for one h at 60 o C. The mixture was then poured into ice cold water, filtered and washed with distilled water many times until it was free from acid.Nitration was achieved at this step.Nitro group on the polymer was then reduced to amino by refluxing it in the presence of stannous chloride (40 g), conc.hydrochloric acid (50 mL) and distilled ethanol (60 mL) for 12 h at 70 o C. The resultant amino polymer was treated with 100 mL of 2 M hydrochloric acid for 30 min to remove stannous chloride and then washed with distilled water to remove excess of hydrochloric acid.The polymer was then suspended in followed by the addition of 2 mL of 1 M sodium nitrite with constant stirring for 30 min for diazotization.The diazotized resin was filtered, washed with ice cold water and was then coupled with α-nitroso β-naphthol (αNβN) by stirring it for 3 hours in 100 mL of 3% α-nitroso β-naphthol solution prepared in distilled ethanol.After coupling the chelated resin was washed with ethanol to remove any unreacted α-nitroso β-naphthol.Chelation with 8-hydroxy quinolone (8HQ) was also done using the same procedure.The proposed reaction mechanism is given in Figure 1.

Batch adsorption experiments using simple and chelated resins
For the adsorption of Cu(II) from aqueous solutions, 100 mg of simple and chelated resins were taken in separate beakers.To each beaker 50 mL of 10 µg mL -1 of Cu(II) solution was added.The pH of each solution was adjusted to the required pH by adding suitable volume of Britton Robinson buffer and was allowed to equilibrate for 60 min.Then it was filtered and the filtrate was collected in a 100 mL volumetric flask and diluted up to mark.Desorption of the adsorbed Cu(II) was carried out with 10 mL mixture of nitric acid and hydrochloric acid (0.5: 4 M).The residual metal ions concentration in the filtrates was determined using flame atomic absorption spectrophotometer (FAAS).Adsorption capacity (q e ) of Cu(II) and percent adsorption was calculated according to the Equation 1 and 2: In Equation 1and 2 q e is the amount of Cu(II) adsorbed on the adsorbent (mg g -1 ), C i and C f represent the initial and equilibrium concentrations of Cu(II) in µg mL -1 , respectively; V is the volume of Cu(II) solution (mL) and m is the amount of adsorbent (g).

Characterization of simple and chelated IR-αNβN and IR-8HQ
In order to confirm the chelation of Amberlite IR-120, various characterization studies using FTIR, SEM and TGA were conducted.The FTIR spectra of simple and chelated Amberlite were igure 2 and 3, respectively.On comparing the spectra, it was clear that in case of chelated Amberlite IR-120, new IR bands have appeared.These bands are at (N=O), 1348 cm -1 for (N-H) and δ(N-H), 1626 and 1383 cm -1 (C=O) and 1540 for N=N group [17].Thus FTIR analysis indicated the successful chelation of Amberlite FTIR spectrum of modified Amberlite IR-120.43 120, various characterization studies using Amberlite were respectively.On comparing the spectra, it was clear 120, new IR bands have appeared.These bands are at d 1540 for N=N group [17].Thus FTIR analysis indicated the successful chelation of Amberlite  The surface morphology of simple and chelated Amberlite IR-120 was investigated by SEM at 15000X magnification.SEM images are shown in Figure 4, 4a and 4b.On comparing these images, it can be seen that the surface of Amberlite IR-120 has been modified and has become smoother after chelation with α-nitroso β-naphthol and 8-hydroxy quinolone indicating the successful chelation.
The thermal stability of simple and chelated Amberlite IR-120 was investigated using TGA and is shown in Figure 5.The TGA curve of IR-120 showed two step mass losses up to 400 °C.In the first step the mass loss was 18% at 310 °C which may be due to the loss of adsorbed water.In the second step the mass loss was 52% with greater rate at 410 °C.In case of IR-αNβN, the TGA curve showed two step mass losses.In the first step the mass loss was 20% at 300 °C and in the second step at 360 °C, the mass loss was 60%.Similarly the TGA curve of IR-8HQ showed two step mass losses.In the first step the mass loss was 20% at 380 °C and the mass loss in the second step was 40% at 450 °C.The TGA study shows that skeletal structure of IR-8HQ is strongly linked as compared to IR-120 or IR-αNβN.

Effect of pH
The solution pH is an important factor which affects the adsorption phenomenon.In order to see the effect of pH on adsorption of Cu(II) on these three adsorbents, pH of the solution was varied from 2.0 to 8.0 using Britton Robinson buffer and the results are shown in Figure 6.The adsorption process strictly depends upon solution pH.It can be seen from the figure that maximum adsorption, 41% and 95% was observed at pH 6 for IR-120 and IR-8HQ, respectively while in case of IR-αNβN maximum adsorption (96%) was observed at pH 7. At lower pH, adsorption was less which may be due to competition between H + and the metal ion for the binding/exchange sites.The adsorption decreased at higher pH owing to the formation of hydroxide of the metal ions.

Effect of contact time
Contact time is another important parameter in adsorption process; therefore, the effect of contact time on adsorption behavior of Cu(II) was investigated in the range of 5 to 70 min and the results are shown in Figure 7. Generally, adsorption increases with increase in contact time and the same results were also observed with adsorption of Cu(II).This may be due to the availability of adsorption sites on adsorbent for the target metal ions.As the adsorption process continues, the adsorption sites become occupied and the adsorption process becomes slow.

Adsorption kinetics
The adsorption kinetic data of Cu(II) were fitted in four common kinetic models like pseudo first order, pseudo second order, intraparticle diffusion and Elovich equations.The model for pseudo first order is expressed by Equation 3: where q e is the amount of metal ion adsorbed (mg g -1 ) at equilibrium, q t is the amount of metal ions adsorbed (mg g -1 ) at any given time (min) and K 1 is the pseudo first order reaction rate constant for adsorption (min -1 ).The model for pseudo second order is expressed with the help of Equation 4: where q e is the amount of metal ion adsorbed (mg g -1 ) at equilibrium, q t is the amount of metal ions adsorbed (mg g -1 ) at any given time (min) and K 2 is the pseudo second order reaction rate constant for adsorption (g mg -1 min -1 ).
The constant values of first order kinetics K 1 , q e , R 2 and the second order kinetics K 2 , q e and R 2 were calculated from the slope and intercept of linear line of each model respectively and are given in Table 1.It was observed that not only the correlation coefficient values of the pseudo second order model are higher than that of first order model but the q e values of the second order are closer to the experimental q e values.This suggests that the adsorption might be controlled by pseudo second order model.Table 1.Kinetic parameters of Cu(II) adsorption using IR-120, IR-αNβN and IR-8HQ.
Intraparticle diffusion is a kinetic model which is related to the transfer of adsorbate ions from its aqueous media to the pores of adsorbent.This model is generally expressed by the Equation 5: where C is the intercept and related to the thickness of the boundary layer and K int (mg g -1 min -1/2 ) is the intraparticle diffusion rate constant.The values of these constants were calculated directly from the intercept and slope of the graph and are given in Table 1.The plot of q t versus t 1/2 is not passing through the origin which indicates that intraparticle diffusion is not the controlling step during adsorption of Cu(II) on these three adsorbents and some other mechanisms are involved.The Elovich kinetic equation is used to describe the kinetics of chemisorption on heterogeneous surfaces and is given by Equation 6: where q t is the amount of Cu(II) adsorbed (mg g -1 ) at time (t), α and β are known as the Elovich coefficients, α represents the initial adsorption rate (mg g -1 min -1 ) and β is related to the extent of surface coverage and activation energy for chemisorption (g min -1 ), respectively.The Elovich coefficients were calculated from the linear plot of q t versus ln (t).It may be concluded from Adsorbent qe (mg g -1 ) (exp.)

Pseudo first order kinetic model
Pseudo second order kinetic mode

Intraparticle diffusion model
Elovich model kinetic data that pseudo second order kinetic model is a best choice to describe the experimental data as compared to other models.

Adsorption isotherms
The isotherm models were used to design the adsorption systems of the Cu(II).These models provide a relationship between the amount of Cu(II) adsorbed on the adsorbents and the concentration of Cu(II) in solution at equilibrium.The two most commonly used isotherms models Freundlich and Langmuir were applied to study the adsorption of Cu(II) on the three adsorbents.The Freundlich isotherm is used generally for non-ideal adsorption on heterogeneous surfaces and is expressed by Equation 7: The linear form of Freundlich equation is given in Equation 8: where K F is the Freundlich adsorption isotherm constant (mg g -1 ), 1/n (g L -1 ) and is a measure of the adsorption intensity or the heterogeneity factor whereas n is the measure of the deviation from linearity of adsorption.Its value indicates the degree of non-linearity between solution concentration and adsorption as follows: if the value of n is equal to unity, the adsorption is linear; if the value is below unity, the adsorption is chemical and if the value of n is above unity then adsorption is a favorable physical process.q e is the amount adsorbed (mg g -1 ) and C e is the equilibrium concentration (µg mL -1 ).The Langmuir isotherm is used for monolayer adsorption on a homogeneous surface and is expressed with the help of Equation 9: In a linear form, it is expressed by Equation 10: where C e is the equilibrium concentration (µg mL -1 ), q e is the amount of solute adsorbed per gram of adsorbent, K L and a L are the Langmuir adsorption isotherm constants and are related to the maximum adsorption capacity (L g -1 ) and bonding strength (L mg -1 ), respectively.The theoretical monolayer adsorption capacity is Q o and is numerically equal to K L a L -1 .From the linear form of these isotherm models for Cu(II) adsorption on the three adsorbents, the constant parameters of these isotherms were calculated from the slope and intercept of the linear form of these equations and are given in Table 2.The n values are higher than unity, suggesting that adsorption of Cu(II) on the three adsorbents is a favorable physical process.Maximum adsorption capacities of IR-120, IR-αNβN and IR-8HQ were calculated from the linear form of Langmuir isotherm and were found to be 60.28, 73.89 and 71.50 mg g -1 , respectively.It can be seen that IR-αNβN has maximum adsorption capacity as compared to the other two adsorbents.It may be concluded from these results that the adsorption data of Cu(II) on these three adsorbents were fitted into Langmuir isotherm model with high correlation coefficient (R 2 > 0.99).

Selectivity study
The selectivity study is very important because foreign ions if present along with the target metal ion may form complexes with the chelated adsorbent thus affecting the adsorption of target metal ion.Therefore, the selectivity of the simple and chelated adsorbents was evaluated by studying the effect of selected foreign ions in the range of 50 to 500 µg under optimum conditions on adsorption capacity of the adsorbents for the target metal ion (Figure 8

Desorption studies
Desorption of the adsorbed metal ions is very important not only for preconcentration and subsequent determination of metal ion at trace level but also for the reuse of the adsorbents.For this purpose, various concentration of hydrochloric acid (0.4-5 M) and nitric acid (0.3-1.2 M) were tried and the residual concentration of Cu(II) was determined by FAAS.It can be seen from the Table 3 that desorption with HNO 3 and HCl was ineffective in case of unmodified IR-120 and IR-αNβN while in case of IR-8HQ, 73% and 82% of Cu(II) was recovered with 1.2 M HNO 3 and 4 M HCl, respectively.increase in the adsorption capacity was observed for the IR-8HQ with increase in the sample volume.
Desorption of Cu(II) was also carried out using different volumes for each adsorbent under optimum condition.It was observed that minimum adsorption take place with increase in sample volume in case of simple Amberlite therefore maximum desorption was achieved with increase in sample volume.
The preconcentration factor was calculated for each adsorbent and is given in Table 5.From the table it can be seen that the maximum preconcentration factor of 50 was obtained for IR-8HQ with maximum desorption of Cu(II).

Reusability of IR-120, IR-αNβN and IR-8HQ
Reusability of the simple and chelated resins was studied for Cu(II) with suitable eluting solvents for three consecutive adsorption desorption cycles.IR-120 and IR-αNβN cannot be reuse as % adsorption of Cu(II) decreases to 0.00 whereas IR-8HQ can be reuse five times with a little decrease in % adsorption of Cu(II).

Application/recovery studies of the method
The simple and chelated resins were used for the determination of Cu(II) in aqueous samples collected from river Jhelum and from the selected tube well and tap water.The aqueous samples were spiked with known concentration of Cu(II) and their adsorption and desorption was studied.The results are given in Table 6.The adsorption of Cu(II) was 38-42% for IR-120 and 92-96% for IR-αNβN and IR-8HQ.It was observed that IR-8HQ is more efficient as compared to other two adsorbents with recovery of 86%.

Figure 7 .
Figure 7. Effect of contact time on % adsorption of Cu(II) from aqueous solution using IR-120, IR-αNβN and IR-8HQ.
The adsorption of Cu(II) increased from 17.4% to 41.0% in 60 min for IR-120 and for IR-8HQ, adsorption increased from 38.5% to 95.3%.In case of IR-αNβN, the rate of Cu(II) adsorption was quite fast as compared to other two adsorbents and adsorption reached to 97.8% in first 10 min which indicate that the chelation of Cu(II) with IR-αNβN is faster as compared to IR-8HQ.

Table 6 .
Real sample application for the preconcentration and determination of Cu(II) with IR-120, IR-αNβN and IR-8HQ.